what I'm referring to is one of the following mutations:

mutation
{
    addToUserOnBlog(usersUserId: "cj9ohpnok00zw01196umznk21" blogsBlogId: "cj9og4mlg0ubw0105dtukhnm2") {
      # we don't need this but we need to return something
      usersUserId {
        id
      }
    }
  }

Hey, no that is currently not possible. You will need the id there. But there is a feature request already to enable the usage of other unique fields.